Against the background of climate changes, it is a global aim to decrease the emission of pollutants into the atmosphere. This applies, in particular, to the emission of carbon dioxide (CO2), which collects in the atmosphere, prevents the radiation of heat away from the earth and thus, as a greenhouse effect, leads to an increase in the surface temperature of the earth.
Particularly in the case of fossil-fueled power plants for generating electrical energy, owing to the combustion of a fossil fuel, a carbon dioxide-containing flue gas forms. To avoid or decrease carbon dioxide emissions into the atmosphere, the carbon dioxide must be separated off from the flue gas. Correspondingly, in particular in the case of existing fossil-fueled power plants, suitable measures are being discussed in order to separate off from the exhaust gas the resultant carbon dioxide after the combustion (post-combustion capture).
As a technical implementation, for this purpose, carbon dioxide present in the flue gas after the combustion is scrubbed out of the respective gas stream by an absorption-desorption process by means of a scrubbing medium and an absorbent. For this purpose, frequently amine-containing scrubbing media are used that exhibit good selectivity and high capacity for carbon dioxide.
At all events, these amine-based scrubbing media have a tendency to the formation of nitrosamines owing to the action of nitrogen oxides (NOx) present in the flue gas as a minor component. In the case of non-volatile amines used as scrubbing media such as amino acid salt solutions, the nitrosamines that possibly result are likewise nonvolatile and therefore not relevant to emissions. However, small amounts of volatile amines can form as thermal and oxidative breakdown products (in particular methylamine) and therefore, nevertheless, generate small amounts of emission-relevant components at a low level.
The amine and/or the nitrosamines correspondingly formed as a secondary product accumulate in the scrubbing medium up to the point where a stable equilibrium is established between the rate of formation of the breakdown products and/or the secondary breakdown products (in total degradation products) and also the ejection thereof from the process. The degradation products concentrate with time in the separation process. On account of the high amounts of flue gas introduced into the separation process and the degradation products concentrating, it is possible in this case for these components to be discharged into the atmosphere. These emissions into the environment are to be prevented.
Correspondingly, in the interim, use has been made of scrubbing media in which as active component an amino acid salt is used. Amino acid salts have the advantage that they do not have a significant vapor pressure, and so discharge from the absorber can be avoided. However, in the use of a scrubbing medium having an amino acid salt as active component, degradation of the scrubbing medium cannot be prevented.
Although the breakdown products of scrubbing media containing amino acid salts are for the most part again salt-type components, which likewise do not have a significant vapor pressure, in the case of amino acid salt-containing scrubbing media, a small part of the degradation or breakdown products can consist of ammonia and highly volatile amines, such as, for example, methylamine. Methylamine serves as precursor for the formation of dimethylamine, which in turn forms the emission-relevant dimethylnitrosamine by reaction with NOx. The nitrosamine, and also other amines accumulate in the scrubbing medium and are co-discharged into the atmosphere via the absorber, via the flue gas that has been purified by removing carbon dioxide.
In order to remove these amines and nitrosamines in a controlled manner, to date, the use of what is termed a reclaimer is usual, which ejects soluble contaminants such as amines from the scrubbing medium. However, an avoidance achieved thereby of impermissible emissions of volatile components may only be achieved by an undesirable loss of scrubbing medium.
In order to circumvent this, for example, a purification device for the gas discharged from the absorber can be used. Such a purification device connected downstream of the absorber, however, is associated with high capital costs and structural complexity.
As a further alternative, in addition to the reclaimer, a purification device can be used which is connected downstream of the desorber. For this purpose, a method is disclosed by WO 2013/023918, in which highly volatile degradation products are separated off from an absorbent circuit of a CO2-separation process. For this purpose, a condensate from a condenser connected downstream of the desorber is fed to a purification device in which the condensate is purified by removal of the degradation products present by distillation or by means of an activated carbon scrubber. Such a method, unfortunately, cannot be carried out economically under certain marginal conditions.